The panel is usually a glass panel such as a window panel arranged to be mounted in a car body or a solar panel to be mounted in a frame. In order to mount the window panel in a car body, or to mount the solar panel in the frame, it is provided with a gasket extending along the periphery of the panel. In prior art methods, this gasket is produced directly onto the window panel, either by an extrusion or by an injection moulding process. Such extrusion or injection processes have significant disadvantages.
In the extrusion process, a profiled strand of a reacting polymer system or of a thermoplastic polymer is extruded and deposited onto the edge of the window panel by means of a calibrated nozzle guided by means of an automatic handling device. The reacting polymer system or the thermoplastic material is applied in a pasty or kneadable state, i.e. in a highly viscous state, so that it retains its shape when being applied on the window panel instead of flowing out. According to U.S. Pat. No. 5,362,428 the extruded synthetic resin should have a viscosity in the range of 300,000 to 10,000,000 mPa·s (=cP), and more preferably in the range of 600,000 to 3,000,000 mPa·s (at a shear rate of 1/s) in order to enable to form the synthetic resin into a specified shape or to keep the specified shape until the extruded synthetic resin is cured. An advantage of the extrusion process is that it involves much lower tool costs than the injection moulding process but it has a number of drawbacks. First of all the seam location between the start and the end of the extruded profiled strand must be finished afterwards in an additional process step. Secondly, the extruded strand has a constant cross-sectional profile. Thirdly, it is not possible to extrude around sharp corners so that, at the location of such corners, also an additional finishing step is required. Moreover, the surface of an extruded gasket has only a modest quality.
In order to obtain a window assembly with a gasket of a high dimensional accuracy, U.S. Pat. No. 5,421,940 discloses to extrude a thermoplastic polymer onto the periphery of the window panel and onto an open mould surface extending beyond the periphery of the window panel. A difference with the other extrusion processes is that only a portion of the surface of the thermoplastic material is shaped by the extrusion nozzle, the other part of this surface being moulded against the mould surface. Since the thermoplastic material is partially shaped by the extrusion nozzle, it still should have a quite high viscosity to retain its shape. A drawback of the method disclosed in U.S. Pat. No. 5,421,940 is therefore that the thermoplastic material has to be applied with a sufficiently high pressure onto the mould surface in order to shape the viscous thermoplastic material against the mould surface and against the edge of the panel. In order to be able to apply the required pressure onto the thermoplastic material in the mould, the extrusion nozzle has to be pressed quite strongly against the upper side of the glass panel. In the embodiment illustrated in FIG. 5 of abovementioned patent, wherein not only the front but also the back of the gasket is moulded against the mould surface, the extrusion nozzle has moreover even to fit exactly in the gap between the mould surface and the upper surface of the glass panel. It is clear that the risk on glass breakage is increased by the pressure exerted by the extrusion nozzle onto the glass panel.
Another drawback of the method disclosed in U.S. Pat. No. 5,421,940 is that the pressure which can be exerted onto the thermoplastic material in the mould, is limited since the material is not injected into a completely closed mould. Consequently, compared to an injection moulding process, the surface quality will be worse. Especially when the mould surface shows a fine texture, the thermoplastic material may be too viscous to take over this texture. Moreover, air bubbles may remain present at the interface between the mould surface and the thermoplastic material. In order to guarantee that the surface of the mould shall be completely wetted with the thermoplastic polymer, U.S. Pat. No. 5,421,940 teaches to heat the mould. However, the mould is only heated to a temperature lower than the temperature of the extruded thermoplastic polymer so that the polymer still remains highly viscous. Moreover, by heating the mould, the production cycle time is increased since the polymer must be allowed to set before the window assembly can be removed from the mould.
In a reaction injection moulding (RIM) process, a curable composition is injected under pressure in a closed mould cavity formed around the periphery of the window panel. An advantage of such a RIM process is that low viscous curable compositions may be used (see for example WO 98/14492 disclosing preferred viscosities of polyol and isocyanate blends of between 150 and 2500 mPa·s at application temperature). Advantages of such an injection moulding process are the better surface quality of the gaskets (mainly due to the much lower viscosity of the injected reactive mixture) and the larger design freedom. Important drawbacks of an injection moulding process are however the high tooling costs and also the time and effort needed to make and modify the injection moulds (either when the mould surface is damaged or when a new design is needed). The moulds have indeed to be made of a robust material in order to withstand the relatively high temperature and pressure of the process. This appears for example from EP-B-0 355 209 which discloses to replace the elastomeric seals between the mould surface and the glass pane by a metal ring since elastomeric seals have the disadvantage of imperfect parting lines due to the fact that they tend to deform when too much pressure is exerted thereon. Moreover, a very accurate milling of the mould is needed to avoid glass breakage during mould closure and to finetune contact area between the mould surface and the window panel, so that leakage of the injected material is prevented. These high tooling costs usually impose limits on the production capacity of a RIM process. A further drawback of a RIM process is that an external release agent has to be applied onto the mould surface. This not only involves an extra process step (longer cycle time) but also causes surface defects on the gasket, such as a too low gloss of a high gloss gasket, due to accumulation of this release agent in the mould. Another drawback of a RIM process is finally that the mould cavity has to show a minimum height so that it can be filled completely, i.e. substantially without voids, with the curable composition.
U.S. Pat. No. 6,228,305 discloses still another process to produce a gasket on the periphery of a window panel. In accordance with this process, a window panel is positioned on a lower mould section. Subsequently, a highly viscous adhesive is applied (extruded) onto the edge of the window panel and a highly viscous foam material onto the mould surface which extends along the edge of the window panel. Before the adhesive and the foaming material are completely cured, a pressure is exerted onto the adhesive and onto the foaming material by lowering an upper mould section onto the lower mould section in order to mould the adhesive and the foaming material into the desired form. Due to the fact that the foam material and the adhesive are highly viscous, a considerable pressure will be exerted on the foam material and onto the window panel. A drawback of the method disclosed in U.S. Pat. No. 6,228,305 is therefore that the mould has to be made again of a robust material resulting in the same drawbacks as mentioned hereabove for a RIM process (in particular high tooling costs, risk on glass breakage, . . . ). Moreover, due to the high viscosity of the moulding materials, and the fact that they even will have an increased viscosity when the required pressure is exerted onto the moulding material, the surface quality will not be as good as the surface quality of injection moulded articles, especially not when a fine surface texture has to be taken over from the mould surface.
EP1577080 is considered the closest prior art for the present invention since it discloses a method to produce a panel assembly with a gasket wherein the gasket is formed in an open mould. This allows to use a curable composition that has a dynamic viscosity measured at a shear rate of 1/s, lower than 35000 mPa·s when it arrives onto the mould surface. With such composition, a panel assembly which enables a higher design freedom and a better surface quality of the gasket can be produced compared to the common extrusion process, without involving however tooling costs which are as high as for RIM process. In contrast to the known injection moulding process wherein such less viscous curable compositions are used, the curable composition is not injected in a closed mould but it is applied, either directly or indirectly, on the panel and the mould surface by means of an applicator device moving along the periphery of the panel while applying the curable composition.
Since the curable composition of EP1577080 has a lower viscosity than the polymers used in the known extrusion processes, a better surface quality can be achieved without having to exert high pressures onto the curable composition. In particular, it is possible to take over the surface quality of polished moulds (glossy surface), of structured moulds (for example sandblasted mould surfaces) or of textured moulds (showing for example a leather texture). The curable composition is applied by means of a moving applicator device and in an open mould.
A drawback of forming the gasket in the open mould is that the outer shape and surface texture can only be designed for the part of the gasket that touches the open mould. This limits the application possibilities for the gasket and panel since parts of the shape and texture of the gasket cannot be predefined.